In vapor growth of a nitride semiconductor layer, a sapphire substrate is often used from a viewpoint of stability in crystal growth atmosphere and price. Since the sapphire substrate is not lattice-matched to the nitride semiconductor and has a large difference of thermal expansion coefficient from the nitride semiconductor, a number of crystal defects are easy to occur in a GaN crystal grown on the sapphire substrate. It is considered that two physical properties are related to introduction of the defects. One is strain due to the lattice constant difference between the sapphire substrate and the nitride semiconductor, and the other one is strain due to the thermal expansion coefficient difference between the substrate and a growth layer during a cooling process from a growth temperature to a room temperature.
Those strain cause warpage of the substrate after the crystal growth. The warpage of the substrate affects a following device process. In particular, in a lithography process, parallelism between a mask and the substrate has a great influence on accuracy of pattern transfer, and thus the influence of the warpage is large. Hereafter, further increase of a diameter of the substrate is possibly accelerated for reduction of the device cost, and it is considered that the warpage of the substrate is actualized as a serious problem.